A liquid crystal display device (LCD) makes it possible to serve as a display through regulating transmission of light irradiated from a light source by using a liquid crystal panel composed of a liquid crystal cell and polarizers disposed on both sides thereof. As the polarizers attached to the liquid crystal cell, an absorption type polarizer which is called a light-absorption type dichroic linear polarizer is generally used. A polarizer in which iodine-containing PVA is protected with triacetyl cellulose (TAC) is widely used.
The absorption type polarizer transmits light that is polarized in a direction of transmission axis and absorbs almost all of light that is polarized in a direction perpendicular to the transmission axis. It is pointed out that about 50% of light irradiated from a light source device is absorbed by the absorption type polarizer, and that light utilization efficiency is lowered. Therefore, in order to use efficiently the polarized light in the direction perpendicular to the transmission axis, a configuration, wherein a reflective polarizer called a luminance enhancement film is used between the light source and the liquid crystal panel, has been investigated. As an example of the reflective polarizer, a polymer film that employs optical interference has been investigated (PTL 1 and others).
On the other hand, regarding the polarizer attached to the liquid crystal cell, various kinds of lamination configurations including a reflection display that uses outside light and a transmission display that uses backlight, in which the absorption type polarizer and reflection type polarizer are used in combination, have also been investigated in accordance with kinds and purposes of light used in the display device.
For example, PTL 2 discloses a liquid crystal display device in which an electric field is applied to a liquid crystal layer to vary the retardation value of the liquid crystal and shift a certain amount of the phase difference of the incident polarized light on the liquid crystal layer. Here, as an example of the polarizers used on the both sides of the liquid crystal layer, a reflective polarizer of a planar multilayer structure including three or more layers of films having birefringence, provided on the light source side, and an absorption type polarizer provided on the side opposite to the light source side, interposed by the liquid crystal layer are disclosed.
PTL 3 describes, when an absorption type polarizer and a reflective polarizer provided on the both sides of a liquid crystal cell that includes a liquid crystal between flexible substrates are used, warping is brought about the difference in the amount of expansion and contraction of each polarizer caused by temperature change, and it is proposed to eliminate the warping by combining these polarizers to form a specific laminate configuration. PTL 3 also describes use of a birefringent dielectric multilayer film as an example of the reflective polarizer, and specifically discloses a luminance enhancement film.
However, reflective polarizing polymer films employing a birefringent multi-layer structure, which have been investigated so far (for example, PTLs 4 to 6) have a function of reflecting P-polarized light and transmitting S-polarized light, but their polarization degree has not attained a level equivalent to that of a dichroic linear polarizer.
For example, in the case of a multi-layer laminate film described in PTL 5 and others, in which polyethylene-2,6-naphthalene dicarboxylate (hereinafter, called as 2,6-PEN in some cases) is used for a high refractive index layer and PEN copolymerized with a thermoplastic elastomer or terephthalic acid in an amount of 30 mol % is used as a low refractive index layer, a certain level of polarization performance is achieved by way of: increasing the difference in interlayer refractive indexes in a stretching direction (X direction) through stretching so as to enhance reflectance of P polarization; and, on the other hand, by way of decreasing the difference in interlayer refractive indexes in a direction (Y direction) perpendicular to the in-plane X direction of the film so as to enhance transmission of S-polarized light.
However, when the polarization performance is tried to be enhanced to the level of the dichroic linear polarizer, difference arises, due to the nature of 2,6-PEN polymer, among the refractive indexes in Y direction and among the refractive indexes in the direction of film thickness (Z direction) as stretch develops in X direction; when the interlayer refractive indexes in Y direction are brought into agreement, the difference in the interlayer refractive indexes in Z direction becomes large; accordingly, partial reflection of light that enters in an oblique direction makes hue shift of transmitted light larger.
Therefore, when an attempt is made to put into practical use a liquid crystal display device that is provided with only a polymer film having the above multi-layer structure as one of the polarizers of a liquid crystal cell without an absorption type polarizer combined, achieving both a high polarization degree and elimination of a hue shift in an oblique direction has become an issue.
To solve the problem, the present inventors have, in PTL 7, contemplated the adoption of a reflective polarizer composed of a polymer film having a multi-layer structure, capable of being used as a polarizer disposed adjacent to a liquid crystal cell and of replacing an absorption type polarizer. Accordingly, the present inventors have proposed, by using a certain specific polymer as a high refractive index layer and by uniaxially aligning the polymer, to enhance the polarization performance higher than a having a conventional reflective polarizer having a multi-layer structure and also to improve the hue shift of the transmitted light.
However, although the reflective polarizing film suggested in PTL 7 has achieved a high polarization degree of about 97 to 98%, the film is required to have a well-suppressed hue shift at an oblique angle in an azimuth of 45° between X direction and Y direction (hereinafter, referred to as an oblique azimuth of 45° in some cases), in addition to a further higher polarization performance, in order to be used as a polarizer for a liquid crystal display for a high performance television or personal computer.
[PTL 1]
Japanese Patent Unexamined Patent Application Publication (Translation of PCT application) No. H09-507308
[PTL 2]
Japanese Patent Application Laid-Open Publication No. 2005-316511
[PTL 3]
Japanese Patent Application Laid-Open Publication No. 2009-103817
[PTL 4]
Japanese Patent Application Laid-Open Publication No. H04-268505
[PTL 5]
Japanese Patent Unexamined Patent Application Publication (Translation of PCT application) No. H9-506837
[PTL 6]
International Publication No. WO 01/47711
[PTL 7]
Japanese Patent Application Laid-Open Publication No. 2012-13919